This invention relates to a method of using a detector for gas chromatography. In particular, this invention relates a method of using to a detector of the electron capture type suited for the detection of substances containing a compound with electron affinity.
Electron capture type detectors, which are a kind of detectors for gas chromatography, are suited for the analysis of halogen and nitro compounds and are used for the detection of residual organic mercury, agricultural chemicals or PCB, as well as small amounts of steroid and amino acids as derivatives with electron affinity.
As shown in FIG. 2, a prior art detector of the electron capture type has a small ionization vessel 10 enclosing therein line sources 6 of a radioactive isotope such as .sup.63 Ni serving as negative electrodes and an end part of a column 15 positioned at a bottom section of the vessel 10 to form a nozzle 11 such that a sample, flowing out through this end part of the column 15, can be sent upward within the vessel 10. According to the exemplary design shown in FIG. 2, a pure nitrogen gas can be introduced through purge gas intake routes 12 onto the surfaces of the line sources 6 such that the nitrogen gas thus introduced into the vessel 10 and the .beta.-rays from the line sources 6 generate electrons above the nozzle 11. The electrons thus generated are attracted by an electrostatic field towards a positive collector electrode 13 disposed at an upper part of the vessel 10 and caused to collide with the sample flowing our from the column 15 at the outlet of the nozzle 11, thereby generating negative ions if the sample contains substances with electron affinity. Such negative ions are attracted by and move in the electric field towards the collector electrode 13 but, since these ions move more slowly than the electrons, they have the effect of interfering with the steady flow of current to the collector electrode 13. The detector is designed so as to record the changes in voltage necessary for keeping the current flowing into the collector electrode 13 at a constant rate and to thereby generate a signal indicative of (or proportional to) the concentration of the negative ions.
Prior art electron capture type detectors, as described above, have been in use as highly sensitive and selective detectors with respect to compounds with electron affinity. In order to maintain stability over a long period of time of use, however, they must be cleaned because they become contaminated by the column liquid phase or the components of the samples. Japanese Utility Model Publication Jikkai 1-146158 disclosed an electron capture type detector which allows its interior to be cleaned. Although contamination of an electron capture type detector takes place mainly on the collector electrode and the line sources, contamination of the collector electrode presents a more serious problem in the case of cleanable electron capture type detectors.
When an electron capture type detector is contaminated, its detection sensitivity becomes adversely affected or ghost peaks appear in the chromatograph as shown in FIG. 3, preventing accurate detection. If a radioactive isotope such as .sup.63 Ni is used as line sources 6, furthermore, its handling may be subject to governmental regulations, and specialized knowledge would be required for taking out internal components for cleaning. In other words, one cannot freely disassemble such a detector to clean its internal components.